1. Field of the Invention
The present invention relates to a gas sensor and a method for controlling the same for measuring oxides such as NO, NO2, SO2, CO2, and H2O contained in, for example, atmospheric air and exhaust gas discharged from vehicles or automobiles, and inflammable gases such as CO and CnHm.
2. Description of the Related Art
Recently, an oxygen sensor is widely known, for measuring a specified gas component, for example, oxygen, in which the voltage or the current is controlled to apply it to an oxygen pump based on the use of an oxygen ion-conductive member composed of a solid electrolyte of ZrO2 so that oxygen is pumped in or pumped out under a predetermined diffusion resistance to measure a limiting current obtained during this process (see, for example, Japanese Laid-Open Patent Publication No. 8-271476).
Another sensor is also known, in which a proton pump is constructed by using an oxygen-proton ion-conductive member so that the limiting current is measured on the basis of the same principle as that used in the oxygen sensor to measure H2 and H2O.
A NOx sensor 200 as shown in FIG. 15 is also known, which is used to measure, for example, NOx as a specified gas component.
The NOx sensor 200 is operated as follows. That is, a measurement gas is introduced into a first hollow space 204 via a first diffusion rate-determining section 202. A first oxygen-pumping means 212, which is constructed by an inner pumping electrode 206, an oxygen ion-conductive member 210, and an outer pumping electrode 208, is used to pump out or pump in oxygen contained in the measurement gas in such a degree that the measurement gas is not decomposed. Subsequently, the measurement gas is introduced into a second hollow space 216 via a second diffusion rate-determining section 214. A second oxygen-pumping means 226, which is constructed by a measurement gas-decomposing electrode 218 arranged in the second hollow space 216, an oxygen ion-conductive member 220, and a reference electrode 224 arranged in a reference air section 222, is used to pump out oxygen which is produced by decomposition effected by the catalytic action of the measurement gas-decomposing electrode 218. The sensor measures the value of current which is required to pump out the oxygen.
In other words, the foregoing gas sensors are operated such that the specified gas component is detected by using the ionic current, and the concentration of the predetermined gas is ensured in the internal space of the sensor by controlling the ionic current value.
However, the gas sensors as described above are disadvantageous as follows. That is, when the concentration of the measurement gas is low, the pumping current is decreased. As a result, it is difficult to perform the detection in some cases, and the accuracy is greatly deteriorated by the external electric noise in other cases.
For example, in the case of the NOx sensor 200 shown in FIG. 15, when the NOx concentration in the measurement gas is 10 ppm, the signal level is in a degree of about 0.05 μA. As a result, it is difficult to perform the detection. Further, it is feared that the measurement accuracy is greatly deteriorated due to the external electric noise.
In order to accurately control the oxygen concentration in the second hollow space 216, the present applicant has suggested a NOx sensor 250 as shown in FIG. 16. The NOx sensor 250 comprises an auxiliary pump 252 which is provided for the second hollow space 216. The controlled oxygen concentration in the first hollow space 204 is corrected so that the current, which flows through the auxiliary pump 252, is constant (see, for example, Japanese Laid-Open Patent Publication No. 9-113484 and European Patent Publication No. 0 807 818 A2).
In the case of the NOx sensor 250, the auxiliary pumping current is not more than several μA which is small. Therefore, it has been revealed that the controlled oxygen concentration in the second hollow space 216 cannot be corrected at the desire of a user in some cases.
On the other hand, in the case of the sensors as described above, the limiting current is utilized to control the concentration of the gas component and measure the concentration thereof. Therefore, if the limiting current value is changed, the output is changed. In this context, for example, the limiting current value involves dispersion among individual sensors. At present, in order to correct the dispersion among individual sensors, a shunt resistor is provided, or a voltage divider resistor is provided.
FIG. 17 shows an arrangement of such a countermeasure. When the current, which flows to an oxygen pump 260, is detected by using a current-detecting resistor Ra, the current supply from a variable power source 262 to the oxygen pump 260 is shunted by the aid of an adjusting resistor Rb (shunt resistor).
For example, when the gas sensor has a large limiting current, the shunt resistor Rb is decreased so that the amount of shunt is increased. Thus, the amount of current, which is detected by the current-detecting resistor Ra, is decreased to be a predetermined value. On the contrary, when the gas sensor has a small limiting current, the amount of shunt is decreased so that the current, which is detected by the current-detecting resistor Ra, is adjusted to be the predetermined value.
Another method is also available such that the voltage, which is generated between the both terminals of the current-detecting resistor Ra, is subjected to voltage division by using a voltage divider circuit to obtain a predetermined output voltage.
However, when the foregoing methods (the shunt resistor system and the voltage divider resistor system) are adopted, one extra lead wire is required, in accordance with which it is necessary to use a multiple terminal connector system for connecting the control circuit and the sensor, resulting in a problem concerning the cost.